A suspension system is often used to support and isolate a load, such as a cab or driver's seat, on a truck frame. The vibration environment of the truck frame is an important part of the problem of providing a good vibration isolator. Providing vibration isolation on a truck frame is quite different than providing a wheel suspension system to isolate the frame itself from road disturbances.
The wheel suspension system must accommodate occasional large motions at high speed, such as might be caused by driving over a chuckhole. Because of this, the wheel suspension damper (shock absorber) itself must allow these occasional large high speed displacements without generating large loads that might damage the suspension system or the vehicle itself. In general, since most shock absorbers are built for wheel suspension system use, they embody features that allow these large rapid motions.
On a truck frame, however, matters are different. Chuck holes do not result in large, rapid displacements at the frame, for the wheel suspension system largely isolates those disturbances from the frame. The actual vibrations on the frame, to a first approximation, correspond to a constant acceleration at different frequencies. That is, for a given road and vehicle speed, the spectral distribution of vibration is one of constant peak acceleration during a vibration cycle. The implication of that spectral distribution is that the peak velocity during a vibration cycle decreases linearly with increasing frequency. That is, V.sub.MAX is proportional to 1/FREQUENCY, where V.sub.MAX is the maximum velocity of the frame during a vibration cycle at a given frequency.
Consider the elements of a suspension system. They are simple: a mass supported by a spring, with a damper or shock absorber connected across the spring. This is the description of a seat suspension, a cab suspension, or a wheel suspension.
The conventional way to provide a damper for all of these suspensions is to use a shock absorber with a linear damping characteristic, with other provisions to relieve the forces from a sudden large motion. Such provisions typically are: 1) a relief valve to allow large pressures to escape, and 2) generating most of the damping action during the rebound motion, thereby allowing the shock absorber to move with little damping force when driven upward by the road. Linear damping means that the damping forces are directly proportional to the velocity; e.g., twice as fast motion generates twice the damping force. A system with linear damping behaves proportionally the same way with any amount of motion, and so is the type of damping that is considered in most system analyses, because it simplifies analysis.
The result of the mass and spring combination is that there is a frequency at which the mass will bounce freely on the spring, the natural frequency. Typically, for truck front axle suspensions, this frequency is about 2 cycles per second (Hertz). The damping is typically light, about 10 percent of the amount that is termed critical. Critical damping occurs when a disturbed mass returns to its rest position with no overshoot or oscillation at all. With light damping, any motion that excites the suspension at its natural frequency will be amplified by several times at the suspended body.
Now, consider the case of a truck with a front axle suspension with a natural frequency of 2 Hertz and a seat suspension in the cab with a natural frequency near 2 Hertz. When the front axle suspension is excited (by a big bump, for example) at its natural frequency, the truck frame bounces up and down an amount that can be several times the size of the bump, because of the amplifying effect at the natural frequency. At the same time, the seat suspension in the cab is also amplifying the motion of the frame, which is an amplified motion of the bump. The result is that the driver is often tossed violently about the cab, hitting his or her head on the ceiling and/or being stopped violently by the downward motion stop on the seat. Damage to the driver's head or back may result.
Conventional dampers (shock absorbers) have other problems. Such dampers typically employ oil-filled shock absorbers. To contain the oil, aggressive seals are used that produce excessive friction, thereby making the ride poorer. Damping adjustment is not easily accomplished, although adjustable shock absorbers have been built. Generally, the adjustment control is not convenient to the driver and the adjusted damper characteristics vary widely because of the compromised way that the adjustment is devised within the shock absorber.